MIRI Optics and Focal Plane
The JWST Mid-Infrared Instrument (MIRI) optical light path divides into 2 channels: a spectrometer and an imager. The spectrometer is also configured for integral field unit (IFU) spectroscopy.
Figure 1 shows the Mid-Infrared Instrument (MIRI) field of view in relation to fields of view of other instruments.
MIRI's pick-off mirror, in front of the JWST optical telescope assembly focal plane, directs the MIRI field of view towards the imager. A small fold mirror adjacent to the imager light path picks off the small (up to 8" × 8") field of view of the spectrometer. A second fold in the spectrometer optical path is used to select either the light from the telescope or from the MIRI calibration system.
The optics are configured to place the entrance focal plane just outside the MIRIM housing; this allows the focal plane module (which houses the coronagraph masks and low-resolution spectrometer slit) to be bolted directly to the housing in a very simple interface.
First, the light is collimated. At the pupil image formed by the collimator, a filter wheel holds the following: filters for both the imager and coronagraphs, a prism assembly for the low-resolution spectrometer, a blank for dark current measurements, and a pupil imaging lens. This entrance focal plane is imaged onto the detector using a three-mirror anastigmat camera with separate areas of the detector being dedicated to the imaging, coronagraphy, and spectroscopy functions. The region of the focal plane for each function is selected by a fold mirror close to the telescope focal plane.
Medium-resolution spectrometer (MRS)
Main article: MIRI Medium Resolution Spectroscopy
The MRS provides diffraction-limited integral field spectroscopy (IFU) over the whole wavelength range from 5 to 28.5 μm. This mode consists of two modules: the spectrometer pre-optics (SPO) and spectrometer main optics (SMO).
The SPO spectrally splits the light into the four spectrometer channels and spatially reformats the rectangular fields of view into slits at the entrance of the SMO.
Spectrometer pre-optics (SPO)
Inside the SPO, the light is divided into four different wavelength channels using three dichroics.
Each channel is split, by an additional dichroic chain, into three sub-bands that are observed sequentially by rotation of just two mechanisms that carry both the wavelength sorting dichroics and the dispersion gratings in a very compact and efficient configuration.
Spectrometer main optics (SMO)
The SMO consists of two arms which perform the following three functions: (1) collimation of the output beams of one of the four IFUs, (2) dispersion of the collimated beam with diffraction gratings, and (3) imaging of the resulting spectrum onto one-half of one of the two focal plane arrays.
One of the two spectrometer arms includes the two short wavelength channels (1 and 2); the other has long wavelength channels (3 and 4). Each spectrometer arm uses six gratings (to allow for any combination of two wavelength channels and three sub-band exposures). The dispersed beams are imaged by three-mirror-anastigmat (TMA) camera systems (M1–M2–M3). Folding flats reflect the channel 1 and channel 4 beams such that the combined (channel 1 + 2) and (channel 3 + 4) beam pairs are imaged onto opposite halves of the detectors.
Integral field units (IFUs)
Main article: JWST Integral Field Spectroscopy
The optical path through the IFUs begins with the four toroidal mirrors, which comprise the anamorphic pre-optics (APO) module. The APO re-images the input focal plane (8" × 8") onto the image slicer mirror. Light exits the IFU through individual pupil masks for each beam, then through individual slitlets. Reimaging mirrors behind the slitlets relay the beam to the input of the appropriate spectrometer.